Abstract

Exoplanet discoveries have generated diverse models of plausible terrestrial-type planetary interiors and tectonic regimes. Severe limitations of observable properties require many assumptions about the characteristics of these planets. We present the output of an analytical galactic chemical evolution (GCE) model that constraints one of those properties: radiogenic heating. Earths radiogenic heat generation has evolved since its formation, and the same applies to exoplanets. To make these predictions, we have integrated a GCE model with cosmochemical data for our solar system. Our simulation of the chemical evolution of the interstellar medium in the solar annulus fits the model to the chemistry of our solar system at the time of its formation, and applies the carbonaceous chondrite/Earths mantle ratio to determine the chemical composition of cosmochemically Earth-like exoplanets. In doing so, predictions of exoplanet radiogenic heat productions as a function of age can be derived. The later a planet forms in galactic history, the less radiogenic heat it begins with; however, due to the declining nature of radioactive decay, today, old planets have lower heat outputs per unit volume than newly formed worlds. Due to its long half-life, 232Th continues to provide a small amount of heat in ancient planets, while 40K dominates heating in young ones. By constraining the age-dependent heat production in exoplanets, we can infer that younger, hotter terrestrial planets are more likely to be geologically active. In the search for Earth-like planets, the focus should be made on stars not much older than the Sun.